In communication via satellite to and from a moving vehicle such as a ship or car a “mobile terminal” installed on the vehicle is required. Such a “mobile terminal” is usually composed of one part being installed on a vehicles platform which platform is in a fixed position relative to the vehicle. This platform will hereafter be designated “moving platform” and the part of the terminal that is installed on it is designated EME (external mounted equipment), see also FIG. 1a and FIG. 1b. The “mobile terminal” will also usually include one part being installed at some location near the terminal user e.g. in the wheelhouse on a ship, this part being designated the IME (internal mounted equipment). The IME typically includes handset, PC, modem, interface electronics, power supplies etc. although there is a tendency that more and more of the IME electronics is moved to the EME in order to reduce cost and complexity in both IME and EME.
Satellite communication is becoming more and more popular among mobile subscribers as technology is improved on terminals, satellites and land earth stations. Satellite communication is efficient in remote areas outside the coverage area for traditional land based communication media such as PSTN or cell-phones but lacks the ability to offer high information rates at a competitive low cost. Nowadays, with digital communication techniques being used in almost all communication-systems a good measure for information rate is the bit rate i.e. the amount of information bits transferred per second. Many of the very popular L-band satellite terminals offer a facility to communicate voice, which most often is a low bit rate data transfer, the price being acceptable but still relative high. Also many L-band terminals offer communication of data at a medium speed i.e. 64 kbit per second but at a very inconvenient price.
There is a dramatic increase in the need for true high-speed data transfer via a satellite, at least in the direction from land earth station (LES) via satellite to the “mobile terminal” (also often designated mobile earth station MES, consisting of the above-mentioned EME and IME units), which direction will hereafter be designated the “forward direction”. Here, high-speed date typically means from a few hundred kilobit per second to several megabit per second. The opposite direction in which data is being transferred from the MES via satellite to a LES will be designated the “return direction”. When we say “data being transferred to the MES via a satellite in the forward direction”, we mean data being modulated onto a suitable radio frequency carrier by the LES. This radio frequency carrier is transmitted to a satellite by the LES, which satellite typically is converting the received modulated carrier to a different modulated carrier, which modulated carrier will be given a large amplification and transmitted to the MES. When we say “data being transferred to the LES via a satellite in the “return direction”, we mean data being modulated onto a suitable radio frequency carrier by the MES. This modulated radio frequency carrier is transmitted to a satellite by the MES, which satellite typically is converting the received modulated carrier to a different modulated carrier, which modulated carrier will be given a large amplification and transmitted to the LES. Preferably the above described “high speed data transfer” is utilized also in the “return direction”, but very often a much lower data transfer capability is acceptable.
A data transfer via satellite requires a certain amount of radio frequency bandwidth, the higher the data rate the higher the required bandwidth. In the L-band, the available bandwidth is very limited for which reason bandwidth as a “resource” is very expensive. The L-band is often used for very reliable low to medium speed data rate transfers. The MES equipment and in particular the EME part designed to operate in this band is relative simple and low cost. Global coverage is often seen for L-band systems such as Inmarsat. In the higher frequency bands, such as S, X and K band, bandwidth is more readily available at a reasonable price, but complexity and hence cost of MES equipment, especially for the EME, goes up. Furthermore, global coverage is almost never seen, and coverage is most often limited to a region of the size of e.g. Europe or less.
In U.S. Pat. No. 5,835,057 a satellite communication system is described, in which an antenna assembly is used for receiving Ku-band signals from a first satellite by means of a Ku-band antenna and for transmitting and receiving L-band signals to and from a second satellite by means of a L-band antenna. However, this system is designed to operate in a special case, where the bore-sight axes of the Ku-band antenna and the L-band antenna can be identical, which is the case for the system described in U.S. Pat. No. 5,835,057. This special situation of two or more satellites having the same line of sight from the antennas is the case in North America with at least one of two AMSC satellites and a possible existing Ku-band satellite. The present AMSC system operates via two L-band satellites with about 5 degrees difference in orbital position. However, the system described in U.S. Pat. No. 5,835,057 does not enable simultaneous reception and transmission via two or more satellites, whose difference in orbital angle is much larger than 5 degrees.
Thus, there is a need for an antenna system, which enables simultaneous reception and transmission via two or more satellites, whose difference in orbital angles may vary to an extent, which will not allow for identical bore-sight axes of antennas communicating with different satellites.